A warning sign ("this phone has a third party screen") could be useful, so long as it's just that, a notification in the settings somewhere. Then it's up to me to decide whether I care or not. All it does is give me more information, the judgment on it is my own.
I'd prefer it if it did that for all parts. New third party screen? Notice. New first party screen? Also give notice. Then i know what was repaired or replaced. Even better if it should list when it was replaced, and perhaps the specs of the replacement part.
No but you see we at Google aren't locking down sideloading. It's the individual app developers. With the api we gave them for that express purpose. Totally not us locking stuff down though, so EU please ignore us trying to indirectly close doors in our walled garden?